Technical Abstract:
Vitamin A status in dairy cows has been determined by plasma concentrations of retinol and retinyl palmitate or retinyl palmitate alone, as analyzed by HPLC from a single column assay. Using this type of analytical system, concentrations of retinyl palmitate as high as 750 ng/mL have been reported. Retinol concentrations in plasma range from 150 ng/mL to 250 ng/mL in normal dairy cows. Unfortunately, the alleged retinyl palmitate peak included other compounds in addition to the retinyl palmitate. We recently developed a quantitative assay for retinol and retinyl palmitate which utilizes a 2-column system, resolving retinyl palmitate from other coeluting solutes. The retinyl palmitate peak collected from the first HPLC column was then applied to a C18 column with a mobile phase of 75:20:5 (v:v:v) methanol:chloroform:water. As a test of this new assay for retinyl palmitate and retinol, 10 nonlactating, nonpregnant Jersey cows were assigned randomly to 1 of 3 oral daily treatments of retinyl palmitate for 14 d: 0; 125,000 IU; or 250,000 IU. Retinyl palmitate concentrations in plasma collected during the 14 d ranged from undetectable to 9 ng/mL without dosing and from undetectable to 9 ng/mL with both doses of retinyl palmitate. Retinol concentrations ranged from 180-230 ng/mL without dosing and 225-285 ng/mL with both doses of retinyl palmitate. Retinyl palmitate concentrations of 10 ng/mL are an insignificant proportion of total circulating retinoids since retinol concentrations are normally around 250 ng/mL. Therefore, using retinyl palmitate concentrations in plasma as a diagnostic tool for vitamin A status in dairy cows seems inappropriate.